It has been well established that sometimes cancer clusters within specific families. This has suggested the possibility that some of those families might carry genetic defects which provide susceptibility to specific cancers. Retinoblastoma, an embryonal tumor of the eye represents an extreme example of a tumor which has a dramatic genetic component. Several studies have shown that inactivation at the retinoblastoma gene is probably both necessary and sufficient to initiate retinoblastoma formation. Patients who survive the inherited form of the disease are at risk of developing mesenchymal tumors, melanoma and brain tumors in as high as 10% of the patients before they are 40 years old. Because the product of the retinoblastoma gene, p105Rb, is expressed in all cell types, the obvious question is what accounts for these tissue specific differences in the role of p105Rb. Small cell lung carcinomas virtually all have associated mutations in the Rb gene and yet those tumors do not occur at a significantly increased frequency in patients with the hereditary form of retinoblastoma. In order to identify genes which might predispose to some of the more common adult malignancies, we have focused on one form of hereditary breast cancer. We chose a rare form of hereditary breast cancer which occurs in families with sarcomas (Li-Fraumeni Syndrome). By use of the candidate gene approach we tested which germ line p53 mutations were found in affected family members with Li-Fraumeni Syndrome (LFS). We have found that virtually all of the families with LFS have germ line p53 mutations, and that these tumors have undergone inactivation of the remaining wild-type p53 allele. In order to investigate the role of germ line p53 mutations outside of these rare families, we have begun to investigate other high risk groups. These results indicate that de novo germ line p53 mutations certainly occur in these high risk groups. These findings along with the recognition of the germ line p53 mutations in families with LFS provide clues about the importance of uncovering hidden susceptibilities from germ line tumor suppressor genes not only for the care of patients, but also for understanding the primary events that normally regulate the growth of cells in various tissue.